Imagine your body as a sleek sports car. If you put premium gasoline in the tank, you would expect it to drive beautifully. Yet what would happen if you left the headlights on all night? Never let the engine run more than once a month or two? Didn't change your oil? Surely, the four-wheeled machine would not run as powerfully as it could with proper maintenance.

Now think of your body in the same light. Even if you fueled your tank with nutritious food, you could still feel exhausted with little sleep, little exercise, and a lot of stress. One or a combination of these elements could drain energy reserves. Add any illnesses or medications that can cause fatigue to the mix, and it seems the world is involved in one big heist to steal your engine.

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There is hope. You can rejuvenate your system by following tried-and-true advice from health experts. A lot of their recommendations may seem commonsense. They offer no new, easy, or magical formulas for vigor. Then why follow their suggestions? Perhaps a closer look at four energy thieves can best give the answer.

Energy Thief No. 1: Inactivity

According to Newton's laws of motion, an object at rest tends to remain at rest, and an object in motion tends to remain in motion, unless acted on by an outside force. This is an important concept in energy production, says Sal Fichera, MS, CSCS, an exercise physiologist and owner of Forza Fitness in New York City.

"The body was designed to be in motion," he explains. "When we're not in motion, everything slows down: circulatory systems, energy production. The body, when it's in motion, has to produce energy through various metabolic processes. If we're not in motion, those processes simply slow down to an extreme case where a person just dies and it stops."

Some people are so sedentary that they walk around as if half-dead, with each step an effort, says Fichera. The more active folks, on the other hand, reach a point where movement feels natural.

Don't blame yourself too much if you belong in the first group. The structure and conveniences of today's society make it easy to sit in front of a desk all day, order in meals, groceries and videos, watch hundreds of television shows at any given time, and phone or email people instead of visiting them in person.